1. Field of the Invention
The present invention relates to an ink jet type recording apparatus and more particularly the present invention is directed to miniaturization of such type of recording apparatus.
2. Description of the Prior Art
A typical arrangement of the ink jet recording apparatus having an open ink supply system according to the prior art is shown in FIG. 1.
In FIG. 1, the reference numeral 1 designates a recording head which is formed by using, for example, piezo-electric element. A tank 3 containing an amount of recording liquid 4 (hereinafter simply referred to also as "ink") is connected to the recording head 1 through a supply line 2 to supply the ink 4 to the recording head 1. The tank 3 has a vent hole 5 provided in its upper wall to keep the pressure within the tank at atmospheric pressure. The ink 4 contained in the tank always reaches the tip end of the recording head 1. In response to an electric signal applied to the piezo-electric element of the head 1, the ink is jetted from the tip end of the head, namely from the jet orifice 1A as droplets 6 to effect printing of character, mark etc. on a recording paper. Since there is held a certain proper level difference between the recording liquid in the tank 3 and the recording head 1, the ink consumed as droplets 6 is automatically supplemented to the recording head from the tank through the supply line 2 mainly by the surface tension at the orifice 1A. Therefore, the jet orifice 1A always has a sufficient amount of ink 4 reaching the orifice.
The above mentioned type of ink jet recording apparatus is often incorporated into a portable table computer or typewriter. In this case some problems arise as follows.
As shown in the above, the tank 3 and the recording head 1 are arranged spaced from each other some distance. If the portable calculator or typewriter in which the ink jet recording apparatus has been incorporated is inclined when a man is carrying it, then the level difference between the ink within the tank 3 and that in the orifice 1A can not be kept properly. The meniscus of ink formed at the orifice 1A moves back into the supply line 2 or the ink flows over out of the orifice 1A. In the former case it is required to restore the retrograded mechanics to its original position by using suitable means, for example, by applying a pressure to the ink from the tank side. In the latter case, the flowed ink will make the body of the apparatus dirty. In any case, it troubles the operator very much.
The above mentioned troubles are enhanced by oscillation and/or impact applied to the apparatus. If oscillation or impact is occasionally applied to the main body of apparatus, recording head 1 or supply line 2 or when the recording head 1 runs against the end of print part at a high speed, the meniscus at the orifice 1A is easily broken which results in leaking of ink or retrogradation of ink into the supply line 2. On this occasion, if the level difference between the tank and recording head is at an improper value, it becomes difficult to restore the once broken meniscus to its original position. The ink will continue flowing out of the orifice or the meniscus will further retrograde into the supply line 2. Generation of oscillation and impact force is inevitable for those apparatus in which printing is effected by moving a recording head 1 forward and backward relative to a recording medium such as a recording paper. Under the condition in which the level difference between the recording liquid in the tank 3 and that in the orifice 1A is not at a proper value, the recording head 1 becomes much more sensitive to such oscillation and impact. This is a fatal drawback of the apparatus. For this reason, the reciprocating speed of recording head was limited and therefore high speed printing was difficult to attain.
Another important problem involved in the above mentioned ink jet type of recording apparatus is caused by air bubbles occasionally introduced into the supply line. So long as bubbles are present confined in the supply line 2, no particular trouble will be caused thereby in respect of the function of ink jetting from the orifice. However, when such air bubbles move with the recording liquid through the supply line and enter the recording head 1, the jetting of ink is disturbed by the bubbles. The trouble becomes much more serious in particular when deformation of an electrical-mechanical converter is used as the jet driving source at the recording head 1. In this case, energy generated by the deformation of the electrical-mechanical converter is absorbed by such air bubbles so that transmission of the energy to the recording liquid is hindered. In the worst case, jetting of ink from the recording head is completely stopped thereby and no continuous and stable printing is possible.
To solve the above problems involved in the known ink jet recording apparatus having an open ink feed system, we, the applicant of the present application have already proposed an improved ink jet recording apparatus which is the subject of our prior application, Japanese Patent Application No. 10,189/1979.
FIGS. 2A and B schematically show an example of the improved ink jet recording apparatus.
In FIG. 2, a recording head designated by 10 and an ink tank designated by 11 are united together to form a container 12. The container 12 is fixedly mounted on a carriage 13 which is in turn mounted on a shaft 14 slidably along the shaft. To effect printing on a printing paper, the carriage 13 is moved in the direction across the printing paper. The recording head 10 is constituted of piezo-electric element 15, nozzle part 16, jet orifice 17 and supply tube 18. The supply tube is so bended as to describe an L and extends from the body of the head 10 into the tank 11. Recording liquid (ink) 19 contained in the tank 11 is fed to the recording head 10 through the supply line 18. In this connection, it should be noted that the supply tube 18 is fixed to a wall 20 disposed covering the container 12 so that the supply tube can not be moved by oscillation or impact occasionally applied thereto. The tank 11 has a vent hole 21 to keep the pressure in the tank at atmospheric level. To externally apply electric signals to the piezo-electric element 15 there are provided connectors 22 and 23 connected to the element 15 through signal lines not shown. The apparatus is designed in the manner that the distance between the jet orifice 17 and the free end 18A of the supply tube is adjustable to a suitable value.
The arrangement of ink jet recording apparatus described above has advantages over the aforementioned known apparatus. The recording head 10 and the ink tank 11 are united together to form a container 12 fixedly mounted on a carriage 13. The supply tube 18 having an optimum length for supplying the ink to the recording head is inserted into the tank 11. In the apparatus, no trouble of ink leak from the head 10 or retrogradation of the meniscus into the supply tube 18 can be caused by inclination, oscillation or impact as mentioned above.
However, it has been found that the improved apparatus still has some problems which will be described hereinafter.
For a miniature calculator or other similar apparatus the speed of printing is required to be as high as possible. To satisfy the requirement, the weight of the reciprocating carriage has to be reduced as much as possible taking into consideration the performance of the motor useful for driving the carriage. To reduce the weight load of the carriage the amount of recording liquid to be contained in the ink tank must be limited. The amount of ink actually used in a printer is very small. As an example, in case of a printer of the type in which one character is formed by 5.times.7 dot matrix, it has been experimentally shown that 1 cc of ink is sufficient to print 150-200 thousand characters using a jet orifice having an inner diameter ranging from 50 to 100 .mu.m. Therefore, it may said that if a tank having a capacity of about 3 cc is used as the tank 11, then the electronic instrument provided with the ink jet printing apparatus such as a table calculator with printer can be used for a long time without need of ink supply or tank exchange. However, in a table calculator or in other similar electronic equipment, some problem arises from the ink supply or tank exchange however small the frequency of ink supply or tank exchange may be. It may be possible to set the capacity of tank to such value for which the supply of ink or exchange of the tank is required only one time per half a year. Also, it may be possible to simplify the mechanism necessary for supplying the ink or exchanging the tank to the extent that the work required to supply the ink or exchange the tank is very easy for the operator. However, even under the provision there may be caused some trouble by the supplement or exchange work. It is also a problem that the weight load applied to the carriage by the tank changes with time. In case of 3 cc capacity of tank, the change of weight load reaches about 3 g when the weight of tank full of ink and that of vacant tank are compared. With this change in weight, the carriage driving speed and therefore printing speed also change. This is true in particular when a linear motor is used as the carriage driving motor. Since the printing speed changes with time, it is difficult to maintain the print quality at a desired level.
We, the applicant of the present application have proposed a solution to the above problem in our prior application, Japanese Patent Application No. 103,188/1979 which is the priority application for U.S. application Ser. No. 173,999, filed July 31, 1980, now U.S. Pat. No. 4,342,041, issued July 27, 1982. FIG. 3 schematically shows an ink jet recording apparatus with an ink feeding system operable with pump action as disclosed in the prior application.
In FIG. 3, a recording head is designated by 32 and a subsidiary tank by 33. The head 32 and subsidiary tank 33 are united together to form a head/tank unit 31. The head/tank unit 31 is mounted on a carriage 34 which is in turn mounted on a shaft 35 slide movably along the shaft. The carriage 34 is driven by a motor (not shown). At the lower part of one side wall of the subsidiary tank 33 there is provided an ink inlet 36 with which a flexible supply line is connected. The supply line 37 is connected to another supply line 39 through a fixed point 38. The supply line 39 is connected with an ink outlet 41 provided at the lower portion of one side wall of a main tank 40. Vent holes 42 and 43 are provided on the upper walls of the subsidiary tank 33 and main tank 40 respectively to keep the pressure in the tanks at atmospheric.
The manner of operation of the apparatus shown in FIG. 3 is as follows:
A print signal is applied to the recording head 32 in the manner known per se. In response to the signal the recording head 32 jets ink droplets toward a printing paper (not shown) and also the carriage 34 carrying thereon the head/tank unit 31 is driven by the carriage driving motor (not shown) which is controlled also by the same print signal. As the carriage 34 is driven moving along the shaft 35 in accordance with the print signal, desired character, numeral or mark is printed on the printing paper.
As shown in FIG. 4, the flexible ink supply line 37 swing moves in such manner as to describe an arc about the fixed point 38 as its center when the carriage 34 with the head/tank unit 31 moves leftward and rightward along the shaft 35. Owing to the swing motion of the flexible supply line 37 connected to the head/tank unit there is produced a centrifugal force, that is, a pump action to feed ink to the recording head. Thus, ink is effectively supplied from the main tank 40 to the subsidiary tank 35 in which the ink is gradually decreased by printing.
For the ink jet recording apparatus shown in FIGS. 3 and 4, the pump pressure in the ink supply line extending from the main tank to the subsidiary tank can be found mathematically in the following manner:
For the purpose of illustration, description will be made with reference to FIG. 5 in which the subsidiary tank of the head/tank unit 31 and the main tank 40 are shown to be connected to each other directly by the supply line 37 without any intermediate fixed point.
The head/tank unit 31 moves along the shaft 35 at an angular velocity .omega. (speed Vl). The length of the supply line 37 measured from the main tank 40 to the head/tank unit 31 is l. The cross-sectional area of the supply line 37 is S. The specific weight of the ink is .rho., its gravitational acceleration is g and the speed of the supply line 37 at a distance r from the wall of main tank 40 is Vr.
Under the above conditions, let F denote the force produced by a pressure difference dp between the front side and backside of a minute area dr of the supply line 37 at the distance r and f the centrifugal force produced by the swing motion of the supply line 37 about the main tank 40 and acting on the ink. Then F=f.
On the other hand, F, that is, the force applied to the ink by the pressure difference dp can be represented by the following equation: ##EQU1## Also, the centrifugal force f can be represented by ##EQU2## From F=f, ##EQU3## Therefore, the pressure P can be found by the equation, ##EQU4##
Let P be the pump pressure. Then it will be understood that the pump pressure P is proportional to the square of carriage speed Vl. Thus, every time when the carriage 34 is moved, the recording liquid in the supply line 37 is subjected to a centrifugal force which depends upon the running speed of the carriage. The centrifugal force produces a pumping action to effectively supply the recording liquid to the subsidiary tank 33 from the main tank 40. Therefore, high speed printing can be realized by driving the carriage at a sufficiently high speed enough to overcome the force against ink feed (which is, for example, a composite resistance force formed by the inner diameter and material of the supply line, filter etc.).
As previously noted, the head/tank unit 31 is desired to be light in weight and small in size to attain high speed printing. A preferred arrangement of head/tank unit 31 to satisfy the requirements is shown in FIG. 6.
In FIG. 6, a recording head generally designated by 50 is constituted of piezo-electric element 51, nozzle part 52, jet orifice 53 and supply tube 54. Designated by 55 is a subsidiary tank having a projection 55A. The projection 55A is formed by extending one side wall of the subsidiary tank (the left-hand side wall thereof as viewed in the drawing of FIG. 6) and bending the extension at right angles approximately in line with the upper surface of the subsidiary tank. With the under surface of the projection 55A, it is fixed to the carriage 34. The recording head 50 is fixedly mounted on the projection 55A. Therefore, in this arrangement, the carriage 34 carries the recording head 50 on its upper surface and the subsidiary 55 on its side surface. Compared with the arrangement shown in FIG. 2, it is evident that the arrangement shown in FIG. 6 is smaller in size, lighter in weight and thinner in shape and therefore preferred. Again, vent hole 56 is provided in the upper wall of the subsidiary tank to keep the interior of the tank in communication to the atmosphere. The upper wall of the subsidiary tank 55 may be formed by polyethylene, fluororesin or silicone resin in the form of plate provided therein a number of openings having an inner diameter less than 100 .mu.m serving as the vent hole 56. Also, a membrane filter may be used to form the upper wall of the tank 55.
The subsidiary tank 55 has an ink inlet 57 provided at the lower portion of its one side wall (the right-hand side wall as viewed in the drawing). A supply line 37 is connected to the ink inlet 57 to supply ink to the tank 55.
Since the arrangement shown in FIG. 6 is basically the same as that shown in FIG. 2, the recording head 50 is insensitive to inclination, oscillation and shock. Troubles of ink leak from the head tip and retrogradation of meniscus as previously mentioned can be prevented completely. In addition, the head/tank unit shown in FIG. 6 is smaller in size, lighter in weight and thinner in form than that in shown in FIG. 2
Even when air bubbles are introduced into the supply lines 37 and 39, no trouble can be caused thereby. In the conventional apparatus shown in FIG. 1, such air bubbles can enter directly the recording head. In the improved apparatus shown in FIGS. 2 and 6, however, no bubble is allowed to directly enter the head. In the case of the apparatus shown in FIG. 6, such air bubbles will enter the subsidiary tank 55. Since the tank 55 is provided with a vent filter 56, the bubbles once entered the tank 55 will disappear there and no bubble can enter the recording head 50. Therefore, there is no fear of the jet orifice 53 being blocked by air bubbles. Stable printing is assured. Furthermore, since the subsidiary tank serves also as a buffer to the pump pressure, there is no fear of the ink being flown over out of the orifice. This makes it possible to drive the carriage at a higher speed than in the conventional apparatus shown in FIG. 1.
As seen from FIGS. 3 and 4, the recording apparatus in which ink supply to the subsidiary tank is carried out by means of pump action, has another advantage that the bottom level of the main tank 40 can be positioned lower than the bottom level of the subsidiary tank 33.
All the improvements in ink jet recording apparatus already proposed by our prior applications and described in detail in the above are primarily directed to the prevention of ink leak which often occurs when a man carrys electronic equipment with a printer such as a table calculator with printer in which an ink jet type recording apparatus is incorporated. While the above mentioned improvements are worthy of appreciation, there is another problem to be solved regarding the ink jet recording apparatus. When an ink jet recording apparatus is used as a printer to be mounted in miniature electronic equipment, the apparatus is required to have a compact form as small as possible considering the available space. Hitherto, the location for mounting the main tank has been limited severely because of its positional relation to the recording head. This limitation has constituted an obstacle to further miniaturization of the apparatus.
In the conventional ink jet recording apparatus shown in FIG. 1, the recording liquid is supplied to the recording head 1 from the main tank 3 by the surface tension at the orifice 1A of the head 1. This means that the main tank 3 should be located at a level higher than the recording head 1. Under the condition, it is very difficult to miniaturize the ink jet recording apparatus.